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HeatSpring PV Course Blog

Roof Slope Calculation, 120% Rule breaker Sizing & Calculator on “smart” Phone

From HeatSpring Advanced PV Course Discussion Board

hi Doc ,really enjoying your humor and the way your talking points come through.
Two questions though:this pertains to assignment 16/36,
1)roof slope angle short cut,the inverse tan function is not on my calculator is there another long method of completing that function and 
2)the 120% rule the 50 amps *0.8 where did the 0.8 come from ?
appreciate all the help,ok forgive me one more thing,in the string sizing calculation when I click on the [1/x] function it displays
using your figure of 44.62 X 1/. I am using the calculator on samsung galaxy phone they call it a "smart phone".
Thanks Doc

Roof slope and calculator question:
You can figure out a roof slope with rise and run with a few buttons on the calculator.
If you have trig functions, all you have to figure out how to do is the inverse tangent function, which is sometimes very difficult to figure out. On my apple computer I figured I have to press shift and Tan to get the inverse tangent function. The inverse trig functions give you the angle and the regular trig functions gives you the ratio of sides of a triangle.
Roof slopes in the US are typically done with the rise:run ratio, such as 4:12 being a roof that has 4 units of rise to 12 units of run.
Here is how to do it on a calculator for a 4:12 roof :
4/12=    you get 0.333333
0.333333 inverse tangent    you get 18.4
That’s all there is to it, a 4:12 roof is an 18 degree slope.
Impress a roofer, they will worship you.
I bet your phone does the inverse function, you just have to play with it. My iPhone I have to turn it sideways to get the scientific calculator.
I just did a Google search and it says that you can turn your phone sideways for the scientific functions. Give it a shot and impress people. The word “trig” puts the fear into most people and if you can use trig, some people will think you are a prophet. You can move to Los Angeles and start your own cult.
120% rule 705.12(D)(2)(3)(c) question:
To size the maximum allowable circuit breaker using the 120% rule, forget about the cryptic code, here is the math:
Multiply Busbar x 1.2 to get 120% of busbar
Example: if Busbar is 100A then 100A x 1.2 = 120A
Subtract the main breaker from the busbar
Example: If main breaker is also 100A then 120A – 100A = 20A
In this case 20A would be the maximum breaker that could work for the 120% rule.
A breaker has to be 125% greater than the inverter current
This is the same as saying the maximum inverter current can be 80% of the rating of the breaker.
This is because 80% is the inverse of 125%
1/0.8 = 1.25
1/1.25 = 0.8
Example: the most current a 20A solar breaker can take is 16A
20A x 0.8 = 16A
20A/1.25 = 16A
And an interesting note:
For US residential 240V a 16 A inverter is 3.8kW, which is a very common inverter size, much more common than a 4kW inverter, because the calculation that we just did for a 100A main breaker and a 100A busbar is a very common situation.
Sean White x 1.2
I am the Busbar

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